Fuzzy display on external monitor?

Wondering if anyone else has encountered this problem :
I connected an Acer H233 monitor to use as an extended display with my MB using a monoprice HDMI connector and find the picture quality very poor (fuzzy text) when I set the resolution to 1920 x 1080 (native resolution of the acer). Text is actually clearer when I set the display to lower resolution settings and i operate in mirror mode. Folks at the Apple Store claimed the MB won't support a higher resolution (even when its not in mirror mode) unless the MB screen is not used (clamshell mode?). That doesn't make sense to me since i can clearly select the higher resolution setting, but I can't seem to find anyone else with this issue searching the web. I've read plenty accounts of people having poor quality from erroneously having the mirror mode on or from using inferior VGA cables but none similar to mine. I have tried changing the monitor settings/adjustments but it didn't help nearly enough. Any input would be much appreciated.